1. Field of the Invention
The present invention relates in general to a hydraulic control apparatus for an automatic transmission, and more particularly to techniques associated with a fail-safe arrangement to prevent interlocking of simultaneous engaging actions of some combinations of frictional coupling devices provided in the automatic transmission.
2. Discussion of Related Art
There is known an automatic transmission which includes two hydraulically operated frictional coupling devices of a first group and a plurality of hydraulically operated frictional coupling devices of a second group, and which is arranged to establish a selected one of a plurality of gear positions having respective different speed ratios, by simultaneous engaging actions of one of the frictional coupling devices of the first group and one of the frictional coupling devices of the second group. JP-2000-249219A discloses an example of this type of automatic transmission, wherein a clutch C1 and a clutch C2 constitutes the first group, while a clutch C3 and brakes B1 and B2 constitutes the second group. This automatic transmission is placed in one of first through third gear positions by simultaneous engaging actions of the clutch C1 of the first group and one of the clutch C3 and brakes B1, B2 of the second group, in a fourth gear position by simultaneous engaging actions of the two clutches C1 and C2 of the first group, and in one of fifth and sixth gear positions by simultaneous engaging actions of the clutch C2 of the first group and one of the clutch C3 and brake B1 of the second group.
The publication JP-2000-249219A also discloses a technical arrangement for preventing so-called “interlocking” of simultaneous engaging actions of some combinations of the hydraulically operated frictional coupling devices. This technical arrangement includes (a) a composite-pilot-signal valve operable to generate a pilot hydraulic pressure when the composite-pilot-signal valve receives engaging hydraulic pressures simultaneously generated to engage the two hydraulically operated frictional coupling devices (C1, C2) of the first group, and (b) a fail-safe valve 120 which is arranged to receive engaging hydraulic pressures generated to engage the plurality of hydraulically operated frictional coupling devices (C3, B1, B2) of the second group and the above-indicated pilot hydraulic pressure, and which is operable to prevent application of the engaging hydraulic pressure to a selected one of the frictional coupling devices when the at least two of the pilot hydraulic pressure and the engaging hydraulic pressures generated to engage the frictional coupling devices of the second group. The fail-safe valve cooperates with the composite-pilot-signal valve to prevent the interlocking of the simultaneous engaging actions of the two or more frictional coupling devices which are not normally available due to speed differences of the rotary elements associated with the frictional coupling devices in question but abnormally take place due to an electric failure or defect of solenoid valves or an operational failure or defect of the solenoid valves such as sticking of their spools (operational abnormality due to a foreign matter caught in the valve structure). The solenoid valves are provided to apply and remove the engaging hydraulic pressures to and from the frictional coupling devices. The above-indicated simultaneous engaging actions which cause the interlocking are those of the frictional coupling devices for establishing different gear positions of the automatic transmission. The interlocking would cause an undesirable variation of an output drive force of the automatic transmission, or an excessively large load acting on the friction members of the frictional coupling devices, which results in deterioration of the durability of the friction members.
However, the conventional technical arrangement for preventing the interlocking is arranged such that the fail-safe valve receives the engaging hydraulic pressures of the plurality of hydraulically operated frictional coupling devices of the second group, and therefore undesirably suffers from a relatively large number of hydraulic ports of the fail-safe valve, and an accordingly increased required diameter or length of the fail-safe valve. Therefore, the fail-safe valve tends to have a relatively large sliding resistance of its spool and suffer from an accordingly deteriorated operating response and an accordingly reduced resistance to sticking of its spool. These problems are serious where the number of the frictional coupling device of the second group is relatively large for establishing a relatively large number of the gear positions of the automatic transmission.